Before they've even figured out that beliefs can be wrong? I say parents teach them that beliefs can be wrong with Santa, and teach them that desperate desire to believe is not truth with God.

In a perfect world (and the way I would like to raise my kids) it would be the other way around, because God is more obviously wrong than Santa (Santa might break the rules of physics, but he gives you presents; God breaks the rules and does nothing), and Santa actually rewards belief where God doesn't. So I would introduce them to the God hypothesis, let them test prayer, and decide that beliefs can be wrong. Then try them out on Santa, see if they can find the truth despite being offered presents to believe a falsity.

But the important distinction, the critical distinction, is teaching them. When you teach them about motivated cognition, they've got to have the option of getting it right. If a parent withholds presents from a child because they refuse to believe in Santa, and Christmas comes and goes, and the child sees all their friends with presents and they have none themselves, next year they will lie for presents. Social pressure, parental pressure, just wins at that age. Kids usually start deconverting themselves later than single digits.